Amplifiers produce from an input signal, an output signal having an increased magnitude (i.e., gain). Essentially, an amplifier produces a constant output power at a higher level. Different amplifiers are known and produce various types of gains (e.g., voltage and/or current gain). Amplifiers are rated at a maximum power output for use in different applications. The most efficient output for an amplifier typically occurs when operating at the highest rated output. However, amplifiers are often operated at an average power much lower than the highest rated output in order to achieve a linear output. Therefore, there is a tradeoff between efficiency and linearity.
An ideal power amplifier amplifies an input signal with no waveshape alteration. The ideal power amplifier is therefore characterized as having a transfer function (input signal vs. output signal) which is linear with no transfer function discontinuities. In practice, however, a power amplifier has a transfer function with nonlinear and “linear” regions. Whether the power amplifier is operating in a linear or nonlinear region depends in part on the amplitude of the input signal. For the power amplifier to achieve as near to linear operation as possible, the power amplifier is designed to operate within its linear region given the range of possible input signal amplitudes. If the input signal has an amplitude which causes the power amplifier to operate outside the linear region, the power amplifier introduces nonlinear components or distortion to the signal. When the input signal possesses amplitudes which cause the amplifier to compress, to saturate (no appreciable increase in output amplitude with an increase in input amplitude) or to shut-off (no appreciable decrease in output amplitude with a decrease in input amplitude), the output signal is clipped or distorted in a nonlinear fashion. Generally, an amplifier is characterized as having a clipping threshold, and input signals having amplitudes beyond the clipping threshold are clipped at the amplifier output. In addition to distorting the signal, the clipping or nonlinear distortion of the input signal, generates spectral regrowth or adjacent channel power (ACP) that can interfere with an adjacent frequency.
In wireless communications systems, high power amplification of signals for transmission are commonly encountered with very large peak to average power ratios (PAR). For example, in a time division multiple access (TDMA) system, such as Global System for Mobile Communications (GSM) or North American TDMA, when multiple carrier signals are combined for amplification with a power amplifier, the resulting PAR is about 9–10 dB for a large number of carriers. In a code division multiple access (CDMA) system, a single loaded 1.25 Mhz wide carrier can typically have a PAR of 11.3 dB. For orthogonal frequency division multiplexing (OFDM), multicarrier signals can have a PAR of up to 20 dB. These signals have to be amplified fairly linearly to avoid generating ACP.
Unfortunately, efficiency of the base station amplifier is inversely related to its linearity. To achieve a high degree of linearity, the amplifiers are biased to operate in the class A or “slight” class AB (meaning class AB operation that is closer to class A than to class B). Maximum AC to DC efficiency achievable for class A operation is 50%, whereas that of a class AB amplifier is between 50 and 78.5% (the latter representing the maximum efficiency of a class B amplifier). The closer the particular class AB operation is to class A, the lower the maximum efficiency. For amplifiers employing field effect transistors, the class of operation is set in accordance with the gate voltage applied, which controls the quiescent (idle) drain current. For class A operation, the gate voltage is set so that the idle drain current is approximately in the middle of the range between cutoff and saturation. Class B amplifiers are biased near cutoff, resulting in a rectified drain current waveform. Class AB amplifiers are biased in between the bias points of classes A and B.
Known RF amplifiers have been designed to linearly amplify the peak power of the amplifier rather than the average power (i.e., output only peak power during operation). However, peak power is not continuous, and such designs result in inefficient operation of the RF amplifiers. For example, a 100 watt (W) amplifier may be used to produce 10 W of average power with a peak-to-average power ratio of 10 dB. Operating an amplifier under these conditions can result in an efficiency as low as 5 percent.
Typically, strict linearity requirements in modern wireless communication systems dictate the use of the relatively inefficient class A or slight class AB modes. As a result, significant DC power is dissipated by the amplifiers, thereby generating heat which must be controlled to avoid degrading amplifier performance and reliability. Hence, the use of elaborate heat sinks and fans become a necessary by-product of the high linearity system. Naturally, these measures add to the cost, size and weight of the base station equipment. As the number of wireless communications users continues to grow, so do the number of base stations and the need to keep them small, light and inexpensive.
These very stringent standards have resulted in increased difficulty and complexity in designing efficient RF power amplifiers. Specifically, RF amplifiers must provide spectrally pure (i.e., linear) operation and must be capable of amplifying the peak power content without saturation. During operation of RF power amplifiers, the instantaneous RF envelope power may vary in excess of 10 decibels (dB), which is particularly present in multi-carrier applications. Further, amplifiers in these digital wireless systems must be rated to operate during peak power periods (e.g., during the instantaneous vector sum of multiple RF carrier waveforms), which may occur for only very short durations (e.g., hundredths of nanoseconds). Use of lower rated amplifiers would result in clipping of signals above the highest rated output of the amplifier during those peak power periods and cause interference across communication channels.
Various linearization methods are used to enable the use of more cost-effective and more power efficient amplifiers while maintaining an acceptable level of linearity. Feed-forward correction is routinely deployed in modern amplifiers to improve the linearity of the main amplifier with various input patterns. The essence of the feed-forward correction is to isolate the distortion generated by the main amplifier on a feed forward path. The distortion is provided to a correction amplifier on the feed forward path which amplifies the distortion. The distortion on the feed forward path is combined with the distortion on the main signal path to cancel the distortion on the main signal path.
Predistortion techniques are commonly used to improve the performance of RF power amplifiers. Predistortion techniques distort the input signal prior to amplification by taking into account the transfer function characteristics for the amplifier. The desired amplified signal is achieved from the predistorted input signal by intentionally distorting the signal before the amplifier, so the non-linearity of the amplifier can be compensated.
Predistortion techniques require prior understanding of the response of the amplifier. Both pure digital and RF/digital predistortion implementations use polynomial approximation to predict the amplitude and phase distortion of the RF amplifier from which the predistortion function is determined. Alternatively, the predistortion function can be directly determined using inputs and outputs of the amplifier. In any event, adaptive predistortion systems monitor the signals prior to and after the power amplifier to determine the predistortion function, for example by updating the coefficients of the amplifier characteristics polynomial and calculating the inverse of the amplifier characteristics polynomial. Improving the detection of signals prior to and after the amplifier can improve the determination of the predistortion function, thereby improving the performance of the predistortion system. For example, the improved detection of signals prior to and after the amplifier can improve the determination of the amplifier characteristics function from which a better inverse function can be obtained. In a standard predistortion system implementation, the input signal envelope is detected and used as a pointer to a look-up table which implements the predistortion function. The values of the look-up table are passed to a digital to analog (D/A) converter that modulates or predistorts the input signal to compensate for the distortion of the amplifier. Improvement in amplifier performance is constrained by how well the signals used in the determination of the predistortion system are detected for different input signal levels.
With respect to digital wireless systems, many parts of base stations for these systems (e.g., cellular telephone systems), including RF amplifiers, have decreased in size as a result of improvements in lower power digital circuitry and use of application-specific integrated circuits (ASICs). However, the total size reduction of such systems, and in particular, the component parts, including specifically RF amplifiers, is limited by the amount of heat dissipated by the RF power stages of the RF amplifiers and the associated power supply design. Improvements in the efficiency of the RF power stages in the RF amplifiers would result in further size and weight reductions in the base stations, as well as lower overall power consumption.